In the future, our society and our lives will become even more reliant on networks, so it will be vital that information is transmitted quickly and safely. However, the rise of quantum computers, with their overwhelming computational capabilities, will pose a threat to the security of cryptographic communications. Hopes are high for quantum key distribution as a method of secure encrypted communication. According to information theory, quantum key distribution, which uses the principles of quantum mechanics, is an unbreakable form of encryption. Toshiba is leading the world in developing unique technologies for accelerating and stabilizing quantum key distribution.

In this running feature, we have explained this technology over the course of several articles.

In part 4, the last part of this running feature, we will talk about evolving quantum cryptographic communication technologies, looking ahead to the arrival of quantum computers and the quantum internet. Specifically, we will discuss Twin Field QKD technology, which extends the distances of QKD links, chip-based QKD technology, which makes quantum key distribution devices more compact, and trends related to the quantum internet, the highly anticipated quantum network of the future.

Security technologies for the age of quantum computers

One technology that is anticipated to bring about major societal changes is the quantum internet, a quantum network that connects quantum computers, quantum sensors, and other quantum devices to effectively leverage their capabilities. It is hoped that this technology will make it possible to mutually connect multiple quantum computers, distributing processing across them for overwhelming computational capabilities, and to create diverse applications and systems that use quantum sensors and other devices connected to these networks.

Smartphones, computers, supercomputers, IoT devices including sensors, and various other devices are connected to the conventional internet. However, the quantum states (quantum bits; qubits) input and output by quantum computers and quantum sensors cannot be sent over the conventional internet, so this data must first be converted into digital data. This prevents the leveraging of the full potential of quantum computing. That's why eyes are set on creating a quantum internet, which can effectively use the full capabilities of quantum computers and quantum sensors. We are now in the dawn of the quantum internet -- standardization bodies are already discussing the architecture of the quantum internet and its potential use cases.

However, while hopes are high for the stunning computational capabilities of quantum computers, there are also concerns about cyber-security risks. As we explained in part 1, it has become apparent that quantum computers could instantly decrypt modern codes. This means that no matter how fast future computers may become, it is vital that we have technologies that make it possible for cryptographic keys to be delivered securely, without being eavesdropped on along the way. That's why eyes are turning to quantum cryptographic communication, an important security technology for the age of quantum computing.

Future quantum cryptographic communication technologies

Toshiba's quantum cryptographic communication technology is notable for its high speed, its stability, its compatibility with communications infrastructure, and its interoperability. In part 2 of this running feature, we explained its high speeds, which are essential for handling large volumes of confidential information and its stability, enabling it to securely deliver extremely weak photons bearing cryptographic key information. These advances make quantum cryptographic communication technology more reliable and easy to use. In addition to these technologies, Toshiba is developing technologies for achieving long-distance transmission while minimizing the number of relay nodes in quantum key distribution (QKD) system and improving security. Toshiba is also developing device miniaturization technologies, which will make quantum cryptographic communication easier to use and more accessible.

With conventional technologies, the maximum distance per one QKD link over which quantum cryptographic keys are sent is roughly 100 to 200 kilometers. This is due to a physical limitation placed by optic fibers -- optical attenuation. This means that highly trusted nodes must be used to relay QKD when transmitting them over distances longer than one link.

Toshiba aims to extend QKD distances far beyond the capabilities of conventional systems. That's why we're working on the development of Twin Field QKD technology and satellite QKD systems. Extending the distance per link minimizes the number of trusted nodes that are needed, further improving security.

To minimize quantum key distribution device sizes, we're developing chip-based QKD technologies. These technologies bring multiple optical components together on a single, extremely small chip. This makes it possible to perform quantum cryptographic communication using extremely compact devices.

Let's look at these distance extension and miniaturization technologies in detail.

Technologies for extending distances of QKD links

Toshiba has developed a new QKD protocol and demonstrated that it makes it possible to share cryptographic keys using QKD via optic fiber for distances over 600 kilometers (*1) (*2). It uses a central node to detect photons sent by the devices at the ends of the fiber that are sending and receiving cryptographic keys. This unique Toshiba technology extends QKD distances between the trusted nodes. We call this "Twin Field QKD" technology.

Let's look at the Twin Field QKD procedure (Fig. 1).

  1. Alice and Bob both send a single signal pulse containing the following three random items of information to Charlie.
    * Bit information
    * Basis information
    * Random phase
  2. The pair of signal pulses received by Charlie are detected with detectors 1 or 2. If they are twins, the detection results are as indicated below, depending on the bit information sent by Alice and Bob.
    * The bit information matches: Detected by "Detector 1"
    * The bit information does not match: Detected by "Detector 2"
  3. Charlie uses classical channels (non-quantum) to inform Alice and Bob of the detector that detected the twin.
  4. Alice and Bob use the classical channels to share information on the bases and random phases they chose in step 1. They retain the data they determined were twins and discard all other data. This enables them to obtain each other's undisclosed bit information (see the table at the bottom left of Figure 1).

The bit information shared in this way is used for data encryption, and quantum cryptographic communication is performed. Charlie knows the detection results, but does not know the bit information (that is, the cryptographic key), so even if Charlie's information were stolen, the cryptographic key would not be leaked.

With Twin Field QKD, Alice and Bob, in the end nodes, send signal pulses to the intermediary node (Charlie), and Charlie is capable of detecting single photons. Because of this, if the maximum distance between a light source and a detector is "L," then in conventional one-way transmission the transmission distance would be "L," but with Twin Field QKD, when generating keys at the same speed, the distance between Alice and Bob, going through Charlie (the intermediary node) would be twice that ("2L"), dramatically improving the range of QKD (*3).

In order to enable QKD over non-terrestrial networks (NTNs) including both terrestrial and space communications in future quantum key distribution systems, Toshiba is taking part in the research and development of elemental technologies that can be used to create a satellite QKD system ("Satellite QKD"). Satellite QKD systems require a variety of technologies. Transmitters must be capable of withstanding the cosmic radiation and heat. Receiver must be able to detect extremely weak light from transmitters. Satellite QKD systems and terrestrial quantum key distribution networks must be interoperated. We will conduct further technical development as we aim to accrue a stable of technologies that make it possible to achieve ultra-long-distance communications with links that are tens of thousands of kilometers long (*4).

Technologies for miniaturizing quantum cryptographic communication devices

Now let's take a look at the chip-based QKD technologies we are developing in order to create more compact quantum cryptographic communication devices.

Toshiba is fusing quantum technologies, optical device technologies, and electronic device technologies and combining them with photonic integrated circuit technologies to create chips that offer the functions previously provided by multiple optical components -- transmitters, receivers, and quantum random number generators (Fig. 2). Verification tests using these three chips (QKD chips) and 10 km optic fibers have confirmed the ability to send quantum keys at a speed of 470 kbps for 5.5 continuous days. This is equivalent to the bandwidth necessary for video calls (*5).

The QKD chips designed by Toshiba have various features (Fig. 3).

Figure 3-a shows the QKD chip used in a transmitter (the QTx chip). It contains the following two lasers, a variable optical attenuator (VOA), and an electro-absorption modulator (EAM).

* Primary laser: Generates long phase-modulated optical pulses
* Secondary laser: Uses spontaneous emission noise to generate short pulses with random phases

The QTx uses naturally occurring radiation noise, so it eliminates the need for additional dedicated phase modulator and QRNG. It also uses an EAM (approx. 0.3 mm) that is smaller than a Mach-Zehnder modulator (4 to 5 mm), enabling QTx chip sizes of just 1 mm x 6 mm.

Figure 3-b shows the QKD chip used in a receiver (the QRx chip). This chip contains a Mach‐Zehnder interferometer (MZI), delay line, and phase modulator. It controls excess losses and thermal phase fluctuations, and minimize insertion losses to 4.5 dB, lower than that of other QRx chips (*5).

We are also working on integrating QKD modules containing these chips (Fig. 2) with digital signal processing (DSP) modules that rapidly perform digital information processing known as "key distillation" to develop chip-based QKD systems.

Quantum communication technologies which can be applied to the quantum internet

As mentioned in part 3, progress is being made in the international standardization of quantum key distribution network (QKDN). In addition to this, discussion has begun regarding quantum internet architecture and quantum internet use cases in the Quantum Internet Research Group (QIRG) (*6), part of the IRTF, which is a sister organization to the IETF international internet standardization body. The Quantum Reference Architecture Model for Industrialization (QRAMI), created by the Quantum Strategic Industry Alliance for Revolution (Q-STAR) with the aim of achieving industrialization through quantum technologies, includes a variety of technologies related to quantum communication.

Propelled also by these efforts, a great deal of technology is being investigated around the world. This includes the research of quantum relay technologies for quantum networks to deliver quantum states to distant locations, quantum entanglement generation technology and quantum teleportation technology for quantum relaying, quantum memory technology for temporarily storing quantum states, and quantum interface technology for connecting to quantum networks.

In order to realize the quantum internet of the future, various quantum communication technologies will be needed, including the ICT that supports the modern internet. QKD is positioned as the first step of building the quantum internet.

Figure 4 shows a conceptual image of the relationship between quantum cryptographic communication, the quantum internet, and the modern internet. The internet transmits digital data, but the quantum internet is a network that transmits quantum states. The quantum internet is envisioned as a network used and operated in parallel with the modern internet through the advancement of quantum cryptographic communication technologies and the connection of quantum computers, quantum sensors, and other quantum devices using quantum relay technologies (*8)

QKD technology will continue to be used and refined as a robust security technology that counters threats of quantum computers. Technologies based on QKD technology will advance, creating quantum communications technologies such as quantum relay technologies, eventually evolving into quantum internet technologies.

Toshiba has created, or is in the process of creating, many technologies that could be applied to the quantum internet, such as quantum entanglement generation technologies and quantum relay technologies. We will continue to contribute to the creation of the quantum internet by conducting R&D and standardizing technologies related to quantum cryptographic communication and quantum information relay.

In these four articles, we have explained Toshiba's quantum cryptographic communication technologies and the quantum communication technologies of the future. Toshiba has applied quantum cryptographic communication technologies to various fields, such as medical, industry, and finance, and conducted PoCs in Japan and abroad. We will continue to develop and conduct PoCs of state-of-the-art technologies, creating countless technologies that support the security of our networked society and providing customers with an even richer selection of services.

Reference materials
*1 M. Lucamarini, et al., “Overcoming the rate-distance limit of quantum key distribution without quantum repeaters”, Nature, vol 557, pp. 400-403, 2018

*2 IEICE News Commentary, “Secure Quantum Key Distribution over 500 km of Optical Fiber”, The Journal of IEICE Vol. 101 No.12, Dec. 2018

*3 Mirko Pittaluga et al., “600-km repeater-like quantum communications with dual-band stabilization”, Nature Photonics volume 15, pages530–535, 2021

*4 A. Mamiya et al., “Satellite-based QKD for Global Quantum Cryptographic Network Construction”,
IEEE International Conference on Space Optical Systems and Applications, IEEE, Mar. 2022

*5 Taofiq K. Paraïso et al., “A photonic integrated quantum secure communication system”, Nature Photonics volume 15, pages850–856, 2021

*6 Quantum Internet Research Group (QIRG)

SCIENCE, VOL. 362, NO. 6412, 2018

*8 Jessica Illiano et al., “Quantum Internet Protocol Stack: a Comprehensive Survey”,arXiv, 22 Feb. 2022

Some of these works were partially funded by the EU through the H2020 project, OpenQKD.
This research and development includes the result of “Study and Development of Satellite based QKD and Cryptographic Technology for Global Quantum Cryptographic Network Construction” (JPJ010277) in "Study and Development Project of ICT Priority Technology” (JPMI00316) of the Ministry of Internal Affairs and Communications.
Some of these works were funded by the Innovate UK Collaborative R&D Project AQuaSeC, through the Industrial Strategy Challenge Fund.
Some of these works are based on results obtained from the project JPNP20017 commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Keisuke Mera

Research Scientist
Computer and Network Systems Laboratory
Information and Communications Platform Laboratories
Corporate Research & Development Center
Toshiba Corporation

Since joining Toshiba, Keisuke Mera has been involved in the development of technologies related to IPv6, an international standard on the internet, and the research and development of communications systems for electric power systems and buildings. He is now engaged in the research and development of quantum communications technologies.

  • The corporate names, organization names, job titles and other names and titles appearing in this article are those as of July 2022.

>> Related information

Related articles